The background of the invention lies in the problems which are encountered in measuring traffic flow on roads, using automatic equipment. Such equipment employs a sensor placed in or on a road surface, which produces an output signal in response to passage of a complete vehicle, or of a vehicle wheel, over the sensor. The equipment also includes means for counting these signals so as to provide an indication of the volume of traffic that has passed over the sensor. Various kinds of sensors have been used, including pneumatic tubes and inductive loops. A deformable tube is secured across the road surface in the path of the traffic to be counted and each time a vehicle wheel passes over the tube a pulse of increased pressure occurs in the tube as it is deformed. The pressure pulse is applied to a pressure-sensitive electric switch which generates an electrical pulse which serves as the input signal to the counting equipment.
It has been a significant problem with such equipment, whatever type of sensor has been employed, that it cannot distinguish, from the output signal, in which direction a vehicle has passed over the sensor. This problem does not arise on a single-direction roadway such as a motorway lane, where all the vehicles will necessarily be moving in the same direction, but it arises to various degrees in other situations. For example, on a road carrying a single lane of traffic in one direction and a single lane of traffic in the opposite direction, a sensor can be placed in one lane only in an attempt to measure the traffic flow in only one direction, or separate sensors can be placed in the two lanes in an attempt to separately measure the traffic flows in the two opposite directions; however, there are always a significant number of occasions when vehicles are travelling on the wrong side of the road, at least to a sufficient extent to operate the wrong sensor, such as when overtaking. The result is that the count of vehicles derived from a sensor includes some which were not actually travelling in the direction which that sensor was intended to sense for. The problem is even more significant in a traffic lane which has no preferential direction, such as the centre lane of a three-lane two-direction road, or a narrow country road, in both of which cases vehicles travelling in either direction will use the same part of the roadway. In these situations, all that can be obtained is a total count from which no information as regards the individual flows in the two different directions can be obtained.